1. Field of the Invention
This invention relates to a magnetic reproduce head, and in particular to a magnetoresistive :reproduce head.
2. Description of the Prior Art
As magnetic recording technology continues to push a real recording density limits, magnetoresistive (MR) reproduce heads appear to be the technology of choice. Recent U.S. Pat. Nos. 5,084,794 and 5,193,038 disclose dual magnetoresistive (DMR) reproduce heads which offer improved high linear density performance compared to conventional shielded magnetoresistive (SMR) heads, as well more robust operation and simpler fabrication. U.S. Pat. Nos. 5,084,794 and 5,193,038 are hereby incorporated by reference. Until very recently, virtually all past magnetoresistive sensor/heads, including the DMR design, have been based on the physical phenomenon of anisotropic magnetoresistance (AMR) in Permalloy (NiFe) thin films. U.S. Pat. No. 5,309,303 discloses a magnetoresistive sensor employing the relatively recently discovered "spin-valve" (SV) effect, which is fundamentally distinct from the AMR effect. Sensors or heads based on spin-valve technology can potentially yield significantly greater intrinsic sensitivity and signal levels than any design of conventional the AMR-based sensor or head.
Ideally the basic SV sensing element is a trilayer film of two thin-film magnetic layers sandwiching a very thin non-magnetic conductor. Referring to FIGS. 1a, 1b the basic SV sensor of the prior art consists of the two magnetic layers 10, 12 of thicknesses t.sub.1 and t.sub.2 respectively, separated by a nonmagnetic conductive spacer 14 of thickness t.sub.g, all deposited on a substrate 11. It is to be noted that this SVMR sandwich of magnetic layers 10, 12 and spacer 14 corresponds to the single magnetoresistive film of the prior art AMR sensor. In general, the individual magnetic layers 10, 12 may be either single or multiple layers generally of Co, Fe, and/or Ni; the conductive spacer is generally Au, Ag or Cu. Due to the "spin-valve" effect, the resistivity .rho. of the SV trilayer has a component which depends upon the cosine of the angle between magnetization vectors M.sub.1 and M.sub.2 in the films 10, 12, as will be described below. Depending upon film composition, SV trilayers have been observed to have a magnetoresistive coefficient .DELTA..sub..rho. .rho..sub.0 as large as 8%. This is nearly four times larger than typically found for traditional AMR in NiFe, which accounts for the current substantial interest in SV technology for magnetic recording heads.
The sensor geometry of FIGS. 1a, 1b is designed for detecting magnetic fields H.sub.s along the direction transverse to the SV stripe. Such fields will rotate the magnetization directions, i.e. M.sub.1, M.sub.2 in either magnetic film 10, 12, thereby inducing a change in the magnetoresistive component of .rho.. This in turn changes the net electrical resistance of the SV stripe, creating a voltage change across the terminals of the SV sensor when a constant sense current is passing through the device. In general, the magnetoresistive component of .rho. varies as .DELTA..sub..rho. cos (.theta..sub.1 -.theta..sub.2), where .theta..sub.1 is the angle between the magnetization M.sub.1 and the longitudinal direction of the film 10, and .theta..sub.2 is the angle between the magnetization M.sub.2 and the longitudinal direction of the film 12. Therefore, it is necessary that the films 10 and 12 respond differently to signal fields such that the difference .theta..sub.1 -.theta..sub.2 will vary with the field.
For the SV head as disclosed in U.S. Pat. No. 5,159,513, the magnetization M.sub.2 is "pinned" at .vertline..theta..sub.2 .vertline.=90.degree., and resultantly the magnetoresistive component of .sigma. varies as .DELTA..sub..rho. sin .theta..sub.1. Due to magnetization rotation, sin.theta..sub.1 is proportional to the net transverse signal field H. If .theta..sub.1 .congruent.0 at the zero-field quiescent bias point of the SV sensor, the "sensor output" .varies. "change in .rho." .varies."change in sin.theta..sub.1 " .varies."signal field H", and the SV responds linearly to the signal fields over the maximum possible dynamic range -90.degree..ltoreq..theta..sub.1 .ltoreq.90.degree. prior to the saturation of the film 10. This illustrates why the perpendicular bias state .theta..sub.1 .congruent.0.degree. and .theta..sub.2 .congruent.90.degree. the most desirable for practical application of the SV type magnetoresistive head.
In a practical SV sensor some means is required for pinning the direction of magnetization M.sub.2 of the magnetic layer 12 so that it is substantially perpendicular to the quiescent magnetization M.sub.1 of the magnetic layer 12, which is otherwise free to rotate in response to a magnetic signal field. The preferred means for stabilizing this perpendicular magnetization state as taught in U.S. Pat. No. 5,159,513 entails two distinct features. Firstly, it requires that there be a thickness and/or composition mismatch between the two magnetic SV layers, and secondly, it involves an additional magnetic biasing layer, i.e. the exchanged coupled biasing layer 16 of FIGS. 1a, 1b.
FIG.1b shows the cross section of the SV sensor of FIG. 1a, including deposited current leads 18,20.
A simplified schematic representation of the perpendicularly biased SV of U.S. Pat. No. 5,159,513 illustrates some critical structural and related magnetic features inherent in the design. As taught in the referenced patent, films 10, 12 are Co based alloys and/or NiFe, pinning layer 16 is antiferromagnetic FeMn, and spacer 14 is Cu. The thicknesses of the films 10, 12, 16 and the spacer 12 are as follows: film 10; t.sub.1 .apprxeq.7.5 nm, film 12; t.sub.2 .apprxeq.3.5 nm, film 16; t.sub.3 .apprxeq.10 nm, and spacer 14;t.sub.g .apprxeq.3 nm. All elements are of height L in the transverse direction. Also diagrammed are the transverse magnetic fields present under bias conditions (excluding signal fields), including demagnetization fields H.sub.d and current fields H.sub.j arising from the current density J flowing in the device. There are several possible drawbacks to this design: viz,
a) The design requires a thickness or composition mismatch between the films 10 and 12, and this should be detrimental to the maximum achievable magnitude of (.DELTA..sub.92 /.rho.). This is because the basic spin-valve effect requires sharing of conduction electrons between the two magnetic layers (through the Cu spacer), and this is done most equally and efficiently when the magnetic layers are nominally the same, that is, when the thicknesses of the films 10,12 are equal. In practice, there are several reasons why such a thickness mismatch may be unavoidable. Generally, .theta..sub.1 .varies.(H.sub.j +H.sub.d)/t.sub.1, so that for .theta..sub.1 to be near 0.degree., it is necessary that t.sub.1 be sufficiently large, and that at film 10 the demagnetization and the current fields approximately cancel. The direction of current flow J in FIG. 2 was deliberately chosen such that H.sub.j is antiparallel to H.sub.d at the site of film 10. However, H.sub.d .varies.t.sub.2 /L, while H.sub.j .varies.J(t.sub.2 +t.sub.g), and for the small element heights, L.apprxeq.1 .mu.m required in future high density MR reproduce heads, it is unlikely that H.sub.j will be large enough to cancel H.sub.d at practical maximum allowable current density without t.sub.1 being significantly larger than t.sub.2. Additionally, the exchange pinning strength on film 12 due to film 16 scales as 1/t.sub.2, and achieving sufficient pinning strength to maintain .theta..sub.2 .congruent.90.degree. can require reducing t.sub.2 below minimum thickness requirements on t.sub.1 necessary to avoid saturation of film 10 by signal fields. PA1 b) As taught in U.S. Pat. No. 5,159,513, the last deposited pinning layer 16 is an electrical conductor (as is FeMn) so that sense current from the current leads 18,20 deposited atop film 16 could travel down into the SV trilayer. The presence of a conductive pinning layer shunts sense current away from the SV layers, thereby resulting in a loss of output signal from the device. PA1 c) The most common exchange pinning material used to date, FeMn, is well known to be corrosive, and thus long term durability of the disclosed prior art SV head would be a potentially serious problem. The problem is made worse by the fact that in the present case, the FeMn is in the active area of the SV device, where high current densities and associated Joule heating may accelerate the corrosion. Such heating in the active area is also bad in that the pinning strength of an FeMn exchange-coupling layer can decay significantly with increasing temperature. This temperature problem is exacerbated by the possibility of a slow long term re-annealing of the FeMn in the presence of the magnetic field of H.sub.d +H.sub.j at the site of the interface between films 10 and 12 where H.sub.d and H.sub.j are in the same direction, and oppose the pinned magnetization direction of film 12. Such re-annealing would progressively destroy the transverse pinning of film 12 and render the SV device nonfunctional. PA1 d) The intrinsic linear resolution of a SV reproduce head is not sufficient for a high density recording system, and analogously to the conventional AMR head technology, the presence of additional magnetic shielding as part of the total head design will be required. The shields add cost and complexity in fabricating the head, particularly as the shield/sensor gap spacing must be reduced to accommodate future requirements on increasing storage densities.